1. Field of Invention
This invention relates to a two layer cubic boron nitride (cBN) sintered compact for a cutting tool. More particularly, the invention relates to a cBN sintered compact having two layers, each with a different concentration of cBN therein.
2. Description of the Background Art
Cubic boron nitride is the hardest known material other than diamond. Sintered compact of cubic boron nitride (cBN) is employed in various cutting tools. Recently, sintered compacts having fine cBN particles bonded with metals, or with various ceramics, using a high pressure sintering technique, have become commercially available. When the composite material of the sintered compacts is utilized as an insert in a machining tool, the hard sintered compact layer is placed only on the cutting edge portion. The tool holder to which the hard sintered compact layer is bonded has high rigidity, such as a cemented carbide.
U.S. Pat. No. 4,334,928 of Yazu et al discloses a cBN sintered compact suitable for cutting tools containing about 20 to about 80% by vol. of cBN. The remaining portion is carbide, nitride, boride and silicide of elements selected from groups IVa and Va Periodic Table, with mixtures thereof or solid solution compounds as the principal components. The bonding phase of the sintered compact is continuous.
Japanese Patent Laying-Open No. 62-228450 discloses a hard sintered compact for cutting tools comprising a binder, which comprises about 25 to about 50% by wt. of Al, a compound comprising Ti, such as a carbide of Ti, and from about 4 to about 40% by weight of W, which may be in the form of WC. These components react with cBN during sintering, to produce aluminum boride, titanium boride and the like, which serve to bond the binder and cBN [or another binder].
U.S. Pat. No. 4,911,756 of Nakai et al. discloses a hard sintered compact for tools, comprising from about 50 to about 75% by vol. of cBN and from about 25 to about 50% by vol. of a binder. The binder comprises from about 20 to about 50% by wt. of aluminum, titanium carbonitride, and the like, and from about 4 to about 40% by wt. of tungsten.
U.S. Pat. No. 3,743,489 of Wentorf et al., discloses a cBN sintered compact suitable for cutting tools comprising greater than 70% by volume of cBN. The remaining metallic phase consists of aluminum and at least one alloying element selected from the group consisting of nickel, cobalt, manganese, iron, vanadium, and chromium and mixtures thereof.
U.S. Pat. No. 4,403,015 of Nakai et al. discloses a cBN sintered compact suitable for cutting tools. An intermediate bonding layer is formed from a powder. The bonding layer is comprised of cBN in an amount less than 70% by volume, the residual part principally consisting of a compound selected from carbide, nitride, carbonitride or boride of transition metals of groups IVa, Va and VIa of the Periodic Table, an admixture thereof, or a solid solution compound thereof. After the powder is pressed, or in the state of powder, this bonding layer is placed on the cemented carbide substrate in a thickness less than 2 mm. A powder containing diamond or cBN in an amount in excess of 20% by volume is placed in the bonding layer and the whole compact is then sintered under extreme pressure and temperature.
FIG. 1 is a partial side view of a typical cutting tool of the prior art, having a cutting edge portion of uniform composition of cBN throughout the cutting edge portion. In FIG. 1, a cutting edge portion (20) of uniform composition of cBN sintered composite is bonded to a cemented carbide tool holder (50) by a cemented carbide layer (22). The cutting edge portion (20) is composed of a cutting edge (20c), a rake face (20a) and a flank face (20b). The cutting edge (20c) is a boundary of the rake face and the flank face. FIG. 2, is a partial cross-sectional view of a cutting process using an insert of the tool from FIG. 1. As shown in FIG. 2, a workmaterial (100) rotates at a high rate of speed in the direction of the arrow. The cutting tool is moved into the rotating workmaterial, and a chip (101) is made on the rake face (20a) of the cutting edge portion (20). As cutting continues, crater wear (31) develops on the rake face and flank wear (32) develops on the flank face (20b). The wear on each face is shown in FIG. 3. The crater wear (31) and the flank wear (32) have a boundary of the cutting edge (20c) which contacts with a surface of the workmaterial (100). Notch wear (33) develops at the boundary of the portion in contact with the workmaterial and air. As a result, as cutting continues, partial wear, rather than uniform wear develops on the cutting edge portion. As the wear progresses in a non-uniform manner, it eventually becomes impossible to continue cutting so that the non-uniform wear shortens the practical cutting time.
Accordingly, a need exists for a sintered compact for a cutting tool that has an extended practical cutting time when used as the cutting edge portion of a cutting tool.